The invention relates to a means for controlling the ultrafiltration rate in apparatuses for extracorporeal purification of blood.
It relates in particular to a means for controlling the ultrafiltation rate in hemodialysis apparatuses which comprise a dialyzer having a blood circulation and a dialysis solution circulation, a means for withdrawing ultrafiltrate being provided in the dialysis solution circuit.
In hemodialysis, apart from the metabolism products such as urea, water is also withdrawn from the blood of the patient in the form of ultrafiltrate. It has been found that it is extremely important to control this liquid withdrawal exactly to minimize or avoid the withdrawal problems occurring in the patient. In this connection, in particular the ultrafiltration control is of prime importance. A distinction is to be made between open and closed systems which are realized in single-pass apparatuses.
DE-OS No. 2,644,062 describes a means for automatically controlling a hemodialysis apparatus with open cycle in which in the dialysis solution circuit upstream of the dialyzer a magnetic valve is disposed and downstream of the dialyzer a constant delivery pump is provided.
The magnetic valve is connected to a control unit which controls the magnetic valve in such a manner that a preselected amount of water is continuously withdrawn from the patient during the dialysis. Since however the flow resistance of the dialyzer membrane changes during the dialysis, for example due to clogging and the like, the entire arrangement must be shut down at regular intervals and the flow resistance determined, which firstly is time consuming and secondly above all involves technical problems as regards the apparatus.
This is due on the one hand to the fact that the pump in the dialysis solution circuit carries away the ultrafiltrate whose amount remains small compared with the dialysis solution amount (at the most 1%) and thus cannot be directly determined so that any errors occurring cannot be immediately recognized, and on the other also the fact that the ultrafiltrate itself is not measured until it has already left the blood circulation.
The determination of the ultrafiltrate on the dialysis solution side thus has the disadvantage that for example a slow blood loss to the surroundings, e.g., 10 ml/min, is not detected because the blood loss is not sufficient to effectively change the transmembrane pressure and thus actuate a pressure-sensitive safety device.
The same applies to the withdrawal of blood samples from the blood circuit, which lead to a falsification of the ultrafiltrate determination. In similar manner, an infusion of a replacement fluid into the blood circuit can lead to an unmeasurable change of the ultrafiltrate balance.
DE-PS No. 2,259,787 discloses a hemodialysis apparatus in which liquid is drawn from a closed dialysis solution circulation. However, this apparatus has the disadvantage that the dialysis solution is recirculated so that compared with the single-pass technique the effectiveness of the dialysis is reduced with time due to a decreasing concentration gradient.
A single-pass apparatus having a closed system is known from DE-OS No. 2,838,414 in which balanced dialysis solution which is pumped from and to a balance chamber system is supplied to the dialyzer. Once again, by means of an ultrafiltration pump a predetermined amount of liquid is withdrawn from the closed system and replaced in said system by a corresponding liquid amount which has actually been ultrafiltrated and thus withdrawn from the blood.
A further apparatus known for example from DE-OS No. 3,020,756 determines the ultrafiltration rate from the difference of the incoming and outgoing dialysis solution amounts with the aid of flow meters. The transmembrane pressure can be adjusted so that the ultrafiltration rate corresponds to a predetermined rate.
These known hemodialysis apparatuses have additionally the disadvantage that air bubbles in the dialysis solution can disturb the exact operation of the apparatus because they falsify the ultrafiltration amount. To avoid this, special means are provided, for example air separators.
In addition, these apparatuses involve a high amount of expenditure because the ultrafiltrate amount to be withdrawn of about 2 liters corresponds to a consumption of dialysis solution in the amount of about 120-240 liters, i.e. the ultrafiltration amount is dominated by a relatively large amount of dialysis liquid or solution. Since therefore the amount of ultrafiltrate in the dialysis solution is only about 1-2% the entire apparatus must operate very accurately in the balancing and ultrafiltration in order to avoid endangering the patient.
These known systems thus adjust the ultrafiltration on the side of the dialysis solution without being directly adapted to the conditions of the blood circulation.
DE-OS No. 2,607,022 discloses a hemofiltration apparatus with which the substitution solution is supplied in balanced manner, the pump delivering the substitution solution being controlled with the aid of an ultrasonic flow meter device. Such an ultrasonic flow meter however lacks accuracy because it can determine only a given volume proportion of the liquid flowing in a conduit but not the total liquid amount. Consequently, a change in the flow profile of the liquid or the pulsating flow usual in blood lead to falsifications which make it impossible for such a system to operate accurately. Moreover, such a flow meter cannot be accurately adjusted as is essential.
The ultrafiltration according to this DE-OS has the disadvantage that the ultrafiltration rate can be changed due to clogging of the filter and changes of the blood pressure values in the course of the treatment. Accordingly, with this apparatus a substitution solution is merely supplied without any controlled ultrafiltration being possible, in particular any ultrafiltration independent of a changing blood pressure of changes in the membrane (clogging of the membrane).
In addition, these known apparatuses each comprise an air detector system and systems for detecting blood losses which must be provided independently of the controls of the ultrafiltration and the substitution solution supply. This of course makes the apparatuses expensive, quite apart from the additional sources of possible errors.
It is further to be noted that the known ultrafiltration measuring means or substitution solution control means generally have the disadvantage that they are always designed for a specific method and cannot be combined with each other.